#include <iostream>
#include <stdlib.h>

using namespace std;



class Circle{
public:
    Circle(double Pi,double diameter): P(Pi),d(diameter){}
    double area ( ){return (P*r*r);}
    void setr(double d){ r=d/2;}
    

private:
    double r;
    double d;
    double P;
};


int main()
{
    double diameter,s;
    cin>>diameter;
    double Pi=3.1415;

    Circle a(Pi,diameter);
    a.setr(diameter);
    s=a.area();

    cout<<s;
    system("pause");
    return 0;
}



